The term "Catalyst characterization" is used herein to refer to characterization of catalysts by their pore size, pore volume, pore shapes, compaction, surface area, and other parameters which describe the catalyst structure itself, and also to measurement of the characteristics of the catalyst after use. For example, over time in oil refinery operations some fraction of the pores of a catalyst will become "clogged" with coke or other materials. The amount of this clogging is highly significant to evaluation of the continued efficient use of the catalyst.
Typical catalyst structures vary in overall dimensions from on the order of millimeters to on the order of angstroms. Most prior work in this field has involved investigation of the larger dimensions. Characterization of catalysts having extremely large surface areas and small pore sizes, for example down to five angstroms, is of particular present interest, as these are typical dimensions of zeolitic catalysts. In order to characterize catalysts having such extremely small pores, apparatus for measuring the surface areas of such catalysts is required, because these measurements provide an indication of the pore size and structure. As will appear below, presently available devices do not satisfy the needs of the art.
While the present specification describes the present invention largely in connection with the measurement of the surface area of catalysts, it should be remembered throughout that the techniques discussed herein and the claims appended hereto are not to be so limited.
The basic reference which appears to have precipitated the development of modern surface area determination techniques and accompanying catalyst characterization work is Brunauer, Emmett and Teller (hereinafter "BET"), "Adsorption of Gases in Multi-molecular Layers", J. Amer. Chem. Soc. 60, 309 (1938). In this work there is described the so-called BET equation (eq. A., p. 312) which quantitatively relates the amount of a gas adsorbed on the adsorbent to the partial pressure of the gas in the chamber containing the adsorbent. Specifically, if the amount of gas admitted to the chamber is known, the surface area of the adsorbent can be calculated from the amount adsorbed, which is a function of the partial pressure, using the BET equation.
Physically, as gas enters an evacuated chamber containing an adsorbent, the molecules of the gas are preferentially attracted to the adsorbent by van der Waals forces, such that the gas pressure in the chamber is less than it would have been if a similar amount of gas had been admitted to a similar chamber not having the adsorbent therein. The difference is proportional to the number of molecules of the gas adsorbed on the adsorbent. If a fixed amount of gas is admitted to an evacuated chamber containing an adsorbent at a fixed temperature, the pressure in the chamber will first abruptly rise and will then gradually decrease, until an equilibrium point is reached. At this point, the rates of adsorption and desorption of the gas molecules are equal.
The equilibrium point is a single point on a so-called adsorption isotherm for that particular material, that is, it is a point on a curve which relates the amount of gas adsorbed to the partial pressure of the gas in the chamber at a fixed temperature. If one subsequently admits an additional amount of gas to the chamber, another equilibrium will be reached, determining a second point on the isotherm, and so on. In this way certain constants appearing in the BET equation can be determined. These constants can be used to calculate the surface area of the solid according to procedures well described in the literature. See, for example, U.S. Pat. No. 3,850,040 to Orr et al.
In certain ranges of partial pressures, and given that certain other experimental conditions are controlled, the surface area of an adsorbent can be inferred from a single point on a portion of the isotherm known to be linear. As mentioned, such a point can be determined by admitting a known amount of gas to a sample chamber having a known volume, monitoring the pressure until it reaches a constant value, that is, until equilibrium is reached, and calculating the amount of gas adsorbed therefrom. The BET equation can then be used to derive a value for the surface area of the adsorbent.
One manner in which the amount of gas admitted can be determined is to connect the sample chamber to a chamber of known volume containing the gas to be adsorbed. After a valve connecting the chambers has been opened momentarily to admit a "pulse" of gas into the sample chamber, the amount of gas actually admitted can be determined by noting the change in gas pressure in the chamber of known volume and by application of the ideal gas laws. This is taught by Orr et al.
Another method of determining the amount of gas admitted is to admit the gas at a known flow rate, the flow rate having been accurately calibrated, and measure the amount of time required for a particular equilibrium partial pressure to be reached. A point on the isotherm can then be calculated using the amount of gas admitted, determined from the known flow rate and time. This method is described in U.S. Pat. No. 2,729,969 to Innes. The Innes method was apparently employed in an instrument at one time sold commercially by the Numec Instruments and Controls Corporation of Apollo, Pa., and described in a Numec brochure entitled "Adsorption Flow Apparatus for Particle Surface Area Determinations," which is known to the inventors. Additional points on the isotherm can then be determined using the Innes method as well, provided all else remains equal, so that one can derive the entire isotherm.
The above methods may be referred to as "single point" or "discontinuous" methods, in that only a single point on the isotherm is determined in a given experimental run. Another possibility is to derive more than one of the points on the isotherm in a single run. Multiple ponts on the isotherm can be located by calculating the fraction of the total adsorption sites on the adsorbent covered at any given moment as a function of the net amount of gas admitted to the chamber versus the pressure in the chamber at any given moment. To do so one must ensure that the pressure in the chamber containing the adsorbent is always at equilibrium. Equilibrium occurs when the rates of adsorption and desorption are equal. For the process just discussed to function, the rate of gas flow into the chamber must be less than what is termed herein the "equilibrium rate of adsorption/desorption"; that is, the rate of gas flow into the chamber must be sufficiently slow that if gas flow were terminated no additional net absorption would occur thereafter.
Bosch and Peppelenbos, "Automatic and Low Cost Determination of BET Surface Areas," Journal of Physics E: Scientific Instruments, 10, 605-608 (1977), describe an apparatus in which the latter course is followed. Bosch et al. show use of a capillary of 0.3 meters length and inside diameter of 0.1 millimeter to maintain the rate of flow of gas supplied at atmospheric pressure into an initially evacuated adsorption chamber constant and below the maximum rate of adsorption on the adsorbent. Bosch et al. indicate that the flow rate is assumed to be constant after performance of a calibration run. Id. at page 606. The amount of gas admitted is determined by multiplying the flow rate by the time of flow. Bosch et al. teach that the partial pressure within the evacuated chamber can be assumed to be at equilibrium throughout the experiment. Id. at pages 607-608. Bosch et al. can then determine a number of points on the isotherm in a single experimental run. This is a significant improvement over the method of Innes, for example, which provides a single point on the isotherm per experimental run.
There are certain significant limitations on the apparatus of Bosch et al. Bosch et al. acknowledge that the flow rate provided by their capillary is only constant to within about 0.6% and only over changes in the back pressure between 0 and 8 kPa, that is, between 0 and about 60 Torr. This is insufficient as to both accuracy and useful range. Flow rate varies even further as the partial pressure increases, leading to further inaccuracies. In effect this limits the range of pressures which can be investigated using the Bosch et al device.
U.S. Pat. No. 4,487,213 to Pieters et al provides an improvement on the Bosch et al. apparatus which amounts to an improved flow controller. The Pieters et al. flow controller comprises a capillary tube having two electrically resistive wires wrapped around its ends. A current is passed through both wires to heat gas flowing through the capillary. The gas entering the tube cools the wire at the inlet end of the tube, so that the wire at the outlet end of the tube is warmer than the wire on the inlet end of the tube. The resistance of the wire on the outlet end is therefore higher. This difference in resistance can be measured and used to determine the amount of gas flowing through the tube, as the degree of heating of the gas (and hence the difference between the resistance of the two wires) is proportional to the mass flow therethrough. The resulting flow signal can be used to control a throttling valve to control the gas flow in a simple feedback arrangement.
As mentioned above, the improvement made by Pieters et al. to the science of surface area measurement, as described in their patent, lies in the improvement in the constancy of the gas flow rate. The patent asserts that flow rate control to within about .+-.0.15% over a wide range of pressure differentials is possible using the improved flowmeter. This compares with a flow rate consistency of .+-.0.6% over a narrow range of back pressures as reported by Bosch et al.
It will be appreciated from the description above that the Pieters et al. mass flowmeter requires thermal contact between the gas and the conductive wires by way of the wall of the capillary. This fact is an important limitation on the Pieters et al. flow meter, in that it requires a minimum inlet pressure of about 15 Torr to function. The device therefore is simply not useful for performing experimental work in areas of relatively low inlet pressure. In essence this is due to the fact that at low pressures the walls of the capillary conduct heat faster than does the gas passing therethrough, such that one cannot accurately measure the flow rate of the gas.
Another drawback of the Pieters et al. flowmeter is that it must be calibrated for operation using a given gas, as the thermal conductivity of gases varies widely. Therefore, prior to sample analysis, the gas to be adsorbed is allowed to flow into an empty chamber. The volume of gas admitted can be accurately calculated from the change in pressure over time. This information together with the time required is sufficient information to calibrate the device. In subsequent operations using the particular gas, Pieters et al. must then assume that no significant changes in the relevant experimental parameters take place. Furthermore, of course, the requirement of calibration for each sample gas prevents Pieters et al. from using differing gases for various portions of the analysis, which is sometimes of analytical interest.
There are thus two basic techniques for determining surface areas of adsorbents. The first involves admission of a known amount of gas into a sample chamber at a rate far exceeding the equilibrium rate of adsorption/desorption, such that thereafter the pressure in the chamber containing the adsorbent varies over time; when it stabilizes, the equilibrium pressure has been reached, and a point on the isotherm may be determined. A refinement of this is shown in Innes, wherein the flow rate of gas into the chamber is kept below the equilibrium rate, such that one can calculate the amount of gas adsorbed simply by determining the total amount admitted, which is equal to the unit flow times the admission time, and subtracting the fraction not adsorbed, which is readily determined from the pressure in the vessel. As mentioned this method is only as accurate as the control of the gas flow rate.
The approach taken by Bosch et al. and later by Pieters et al. is somewhat different, in that a continuous flow of gas is permitted and the partial pressure is monitored continuously, such that a number of points on the isotherm can be determined in succession. However, Bosch et al. and Pieters et al. require that the flow can be constant, such that the net volume of gas admitted can be calculated, just as did Innes. It will be appreciated that all of these methods suffer from the sufficient limitation that it is not possible to measure or control mass flow rate accurately. This is particularly the case in the extremely low flow rate and partial pressure ranges of interest in connection with characterization of zeolitic catalysts.
The preceding discussion has focused largely on adsorption of a gas onto an adsorbent. In such experiments the sample is initially located within an evacuated chamber and gas is admitted thereto; if the complete isotherm is desired, gas is admitted until the partial pressure is unity, such that the gas begins to liquefy. One can then plot the relative pressure versus the amount of gas adsorbed, yielding an adsorption isotherm.
Despite the limitations posed by the Pieters et al. flow controller, almost the entire adsorption isotherm for certain gases can be plotted at certain temperatures using their approach. This is because as long as the pressure on the upstream side of the mass flow controller is greater than about 0.02 atmosphere, that is, 15 Torr, its capillary will be "filled" with gas molecules, such that a meaningful measurement will be provided. The flow rate itself can be controlled by a throttling valve located at the outlet end of the mass flow meter. Thus, even extremely low partial pressure regions on the adsorption isotherm can be explored using the Pieters et al. device.
However, the situation is different in desorption. Desorption analysis begins with a sample which, for example, is at least partially saturated with the desorbate. One then gradually removes the atmosphere above the sample. Plotting the relative pressure in the chamber versus the amount of gas desorbed yields a desorption isotherm. Desorption isotherms are extremely significant technically in that they are highly relevant to analysis of pore sizes.
The Pieters et al. patent discloses that the same mass flow controller and in particular the same mass flowmeter can be used in desorption and adsorption studies. However, at the end of the desorption curve, when the partial pressure in the sample chamber drops to less than about 0.02 atmospheres, the Pieters et al. device ceases to function for the reason mentioned above, that is, because the capillary tube of the mass flow meter is not sufficiently filled with gas molecules to provide a measurable temperature gradient. Pieters et al. candidly acknowledge this in their patent, at column 33, line 58.
As indicated above, the conventional BET technique involves measurement of the partial pressure p/p.sub.o of the gas in the chamber containing the adsorbent, that is, the actual pressure p divided by the pressure p.sub.o at which the gas liquefies. The most typical experimental techniques involve cooling the sample in a tube in a bath of liquid nitrogen at atmospheric pressure, i.e. such that p.sub.o is approximately equal to 760 Torr. The limitation on the Pieters et al. flowmeter to relative pressures greater than about 0.02 therefore means that it is functional in desorption analysis, using nitrogen as the desorbed gas, at absolute pressures above about 15 Torr. Hence most of the nitrogen desorption isotherm can be measured, although it should be noted that the lower portions of the isotherm, which cannot be explored with the Pieters et al. device, are of great interest.
However, it is frequently desired to use other gases, such as methane, ethane, hexane, benzene and other hydrocarbons, which have much higher boiling points, as desorbates. If these gases are used in connection with a sample at a convenient temperature, most of the desorption occurs at pressures below the useful range of the Pieters et al. flowmeter. For example, hexane liquefies at approximately 80 Torr at room temperature. Therefore, an extremely significant portion of its desorption curve involves absolute pressures less than 15 Torr at room temperature. Hence, the Pieters et al. method and apparatus are not useful in desorption analysis using such desorbates. Higher carbon number hydrocarbons boil at even higher temperatures, exacerbating the problem.
Nor is performing the analysis at higher sample temperatures a viable alternative; accurate temperature control is difficult, and chemical reactions between the desorbate gas and sample tend to take place (e.g., coking) which fatally impair the accuracy of the experimental data.
Inasmuch as it is clearly an objective of any laboratory equipment to be as versatile as possible, this limitation on the desorption analysis capabilities of the Pieters et al. device is extremely significant. In fact, this may preclude use of the Pieters et al. apparatus in desorption studies of zeolite-range pores, which studies are of great present interest.